The present invention relates generally to the field of intervertebral disc prostheses. More particularly, the invention relates to disc prostheses that have adjustable disc heights, that are readily converted into fusion prostheses and that provide a range of motions that effectively mimic the natural range of motions of a spinal disc.
Intervertebral discs provide elastic support upon compression between adjacent vertebrae in the spine. Damage to these discs, resulting from degeneration and wear, may produce mobility limitations, pain, discomfort and even paralysis. Conventional approaches to the treatment of pathologic, degenerated or ruptured spinal discs include fusion of adjacent vertebrae and disc replacement. In a spinal fusion procedure, the faulty disc may be removed and replaced with a mechanical cage which maintains the proper disc spacing and helps support the load imposed on the spine. Ultimately, bone grows in and around the cage. The vertebrae that are involved in the fusion no longer take part in normal spinal flexing.
Disc replacement is an alternative approach to fusion. Many disc prostheses have been proposed. Unfortunately, the proposed disc prostheses suffer from practical limitations. Ideally, a disc prosthesis will mimic the range of motion of a natural vertebral disc, including side-to-side and front-to-back bending motions, compression in the axial direction and rotation and translation between vertebrae. The disc prosthesis should provide a proper disc height between vertebrae as well as proper alignment of the spine. The disc prosthesis should also be biocompatible, stable and durable. A typical disc prosthesis may be expected to last 30 years or more at approximately 2 million cycles per year.
Unfortunately, disc replacement carries significant risks. For example, distracting the vertebrae to insert or replace a disc prosthesis may injure the vertebrae, the vertebral endplates and the surrounding tissues and ligaments, including the spinal cord, nerve elements and blood vessels. This can be particularly problematic if the disc prosthesis requires repeated replacement or when it is desirable to switch from a disc prosthesis to a fusion prosthesis.
Thus, a need exists for a durable disc prosthesis that minimizes strain on the vertebrae, endplates and ligaments during insertion and replacement while providing a natural disc height and range of motion. Also desirable is a disc prosthesis that may be easily converted into a fusion prosthesis while minimizing the risk of disc extrusion and its potentially lethal complications.